this one has better performance at the cost of complexity

for a toy app like this one the difference is insignificant,
but it might be good to see the two versions and compare them
and based on that be able to make the trade-offs in real apps

the angularjs version does deep watching of the todos array object.
that means that it keeps an in memory copy of the whole array that
is used for dirty checking in order to detect model mutations. For
toy app like this it's totally fine and in fact encouraged practice
since you are trading off a little bit of memory and performance
for simplicity.

However in a large project where you are dealing with array of 100s
or 1000s of large objects you definitely don't want to use this
approach. Instead you want to use a different way of doing the same
thing, which requires more code but is a lot more efficient.